Guide bushes mounted on a column of an automatic lathe to hold a rod-like workpiece rotatably at a position close to a cutting tool are classified into a rotary type and a stationary type. A rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding. A stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding.
A guide bush of either type has a portion having a taper outer surface provided with slits to make the same portion elastic, a threaded portion to hold the guide bush on the column, and an inner surface for holding a workpiece. The inner surface always in sliding contact with a workpiece is liable to be worn and, particularly, the inner surface of a stationary guide bush is worn rapidly.
Therefore, we have already proposed a guide bush wherein a hard carbon film is formed over the inner surface thereof coming in sliding contact with a workpiece when the workpiece is rotated and slid so as to dramatically enhance wear resistance of the inner surface and prevent seizure from occurring between the inner surface and workpiece.
The hard carbon film is formed of a hydrogenated amorphous carbon closely resembling diamond in properties. Therefore, hydrogenated amorphous carbon is also called diamond-like carbon (DLC).
The hard carbon film (DLC film) has a high hardness (not lower than Vickers 3000 Hv), is excellent in wear resistance and corrosion resistance, and has a small coefficient of friction (about 1/8 that of a superhard alloy).
The guide bush having an inner surface to be in sliding contact with a workpiece, coated with the hard carbon film, has wear resistance more excellent than the conventional guide bush having an inner surface attached with a superhard alloy or a ceramic material.
Accordingly, an automatic lathe employing the stationary guide bush provided with the hard carbon film over the inner surface thereof as described above is able to achieve heavy machining, in which depth of cut is large and cutting speed is high, with high accuracy for an extended period of time without damaging the workpiece or causing seizure.
Further, the hard carbon film may preferably be formed on an intermediate layer formed over the inner surface of the guide bush to enhance adhesion between the inner surface and the hard carbon film.
When the intermediate layer is formed of a two-layer film consisting of a lower layer of titanium, chromium or a compound containing titanium or chromium, and an upper layer of silicon, germanium or a compound containing silicon or germanium, the lower layer secures adhesion to the inner surface (alloy tool steel as a substrate metal) of the guide bush, and the upper layer bonds firmly to the hard carbon film. Therefore, the hard carbon film adheres firmly to the inner surface of the guide bush with high adhesion.
The hard carbon film may be formed on a hard lining member of a superhard alloy, such as tungsten carbide (WC), or a sintered ceramic material, such as silicon carbide (SiC), formed on the inner surface of the guide bush. An intermediate layer interposed between such a hard lining member and the hard carbon film will further enhance the adhesion of the hard carbon film.
Even if the guide bush is provided with the hard carbon film over the inner surface thereof as described above, however, the necessity for removing the hard carbon film from the inner surface thereof will arise so as to render the guide bush reusable in case any defect in the hard carbon film is detected during a test after formation thereof, the hard carbon film is damaged after use over a long period of time, or any other trouble is found occurring thereto.
In such a case, it is conceivable to remove the hard carbon film formed over the inner surface of the guide bush by use of a conventional technique such as the plasma etching method.
FIG. 10 is a view for illustrating a method of removing the hard carbon film from the inner surface of a guide bush by use of the plasma etching method.
As shown in the figure, a guide bush 11 with a hard carbon film 15 formed over the inner surface thereof is disposed inside a vacuum vessel 61, having a gas inlet port 63 and an evacuation port 65, and provided with an anode 79 and a filament 81 in the upper part therein, and securely held by insulated holding members 80.
The vacuum vessel 61 is then evacuated by means for evacuation (not shown), removing air through the evacuation port 65. Thereafter, a DC voltage supplied from an anode power source 75 is applied to the anode 79 disposed opposite to the guide bush 11, and an AC voltage supplied from a filament power source 77 is applied to the filament 81 while a DC voltage supplied from a DC power source 73 is applied to the guide bush 11.
Simultaneously, an oxygen-containing gas is fed into the vacuum vessel 61 through the gas inlet port 63, causing an oxygen plasma to be produced within the vacuum vessel 61 so that the hard carbon film 15 formed over the inner surface of the guide bush 11 is removed through etching as a result of oxygen reacting with carbon in the hard carbon film.
With the use of such a method of removing as described above, however, it is impossible to completely remove the hard carbon film 15 formed over the inner surface of the guide bush 11 from the entire region of the inner surface.
This is because with the method of removing as shown in FIG. 10, the plasma entering the center bore 11j of the guide bush 11 from the open end face thereof does not sufficiently reach the innermost region in the center bore 11j, thus failing to produce a uniformly distributed plasma therein.
Consequently, the hard carbon film formed on the inner surface of the guide bush 11, in the vicinity of the open end face thereof, can be removed by etching, but same formed on the innermost side (toward the lower part in FIG. 10) of the inner surface of the guide bush 11 can not.
A method according to the invention has been developed to overcome the problem described above, and it is therefore an object of the invention to provide a method whereby the hard carbon film formed over the inner surface of the guide bush can be removed from the entire region of the inner surface thereof with certainty.